Curable silicone compositions that cure by a hydrosilylation reaction are used as protective agents, coating agents, lens-molding materials, light reflection materials, or the like for optical semiconductor elements in optical semiconductor devices such as photocouplers, light emitting diodes, solid-state imaging devices, or the like. Among these, the compositions used as light reflection materials can be exemplified by a resin composition for a mounting package that incorporates an optical semiconductor element, and comprises a thermosetting type addition reaction-capable silicone resin that has a structure where vinyl groups or allyl groups, and hydrogen atoms are directly bonded to silicon atoms, a platinum-type catalyst as a curing catalyst, and a white pigment (refer to Japanese Unexamined Patent Application Publication No. 2009-21394); and by an addition curable silicone resin composition comprises a vinyl-functional organopolysiloxane having a weight average molecular weight (Mw) greater than or equal to 30,000, an organohydrogenpolysiloxane having at least two silicon atom-bonded hydrogen atoms in a molecule, a white pigment; an inorganic filler other than the white pigment, a platinum metal-type catalyst; and a reaction inhibitor, wherein its cured product has a visible light average reflectance greater than or equal to 80% (refer to Japanese Unexamined Patent Application Publication No. 2011-140550).
These compositions have the following problems in transfer molding, injection molding, or compression molding: low mold filling, facile generation of voids and flashing, and/or a poor mold releasability; also, slow curing rate and a poor workability in the molding operation. In addition, although the cured products obtained by curing these compositions have the advantage of little discoloration by heat and light, the cured products have following problems of high linear expansion coefficients and/or a low mechanical strength at high temperature, as well as the problems of an insufficient light reflectance and great lowering of mechanical strength by heat or light. When such a composition is used to form a light reflection material in an optical semiconductor device in contact with a lead frame, adhesion to the lead frame is poor.
An object of the present invention is to provide a curable silicone composition that has excellent formability and forms a cured product that has little discoloration and lowering of mechanical strength by heat and light, has high light reflectance, and has excellent dimensional stability. A further object of the present invention is to provide a cured product that has little discoloration and lowering of mechanical strength by heat and light, and has high light reflectance. A further object of the present invention is to provide an optical semiconductor device that has a light reflector material that bonds readily to the lead frame.